1. Field of the Invention
The invention relates to the boundary layer refinement of functional components. Objects for which the use thereof is possible and suitable are all functional components composed of titanium or alloys thereof subjected to sliding wear that are loaded at use temperatures less than 500xc2x0 C., are subjected to a high surface pressure, and must have as low a coefficient of friction as possible. The invention is especially advantageous for protecting human implants, in particular with oscillating movement cycles, as well as components from the air and space flight sector that are subjected to sliding wear.
2. Discussion of Background Information
Titanium is an excellent construction material whose high specific strength, chemical stability, and biocompatibility make it especially appropriate for various specialized applications. However, its low resistance to sliding wear and its high coefficient of friction often prevent it from having a broader range of uses.
It is known to produce very wear-resistant boundary layers on titanium by means of laser gas alloying (see, e.g., H. W. Bergmann: xe2x80x9cThermochemische Behandlung von Titan und Titanlegierungen durch Laserumschmelzen und Gaslegieren,xe2x80x9d Zeitschrift fur Werkstofftechnik 16 (1985), p. 392 405).
DE 3,917,211 teaches that it is known to use laser gas alloying to protect joint endoprothesis. For this purpose, the component is melted by the laser beam up to a depth of 0.1 to 0.7 mm and, at the same time, nitrogen is blown into the melt. Because of the high affinity of the titanium to reactive gases, a titanium nitride forms immediately which precipitates out of the melt in the form of needles. After solidification, the boundary layer consists of a metallic matrix of titanium with a different xcex1/xcex2 share in relation to the initial state as well as very thickly embedded titanium nitride needles, some of which are quite coarse. The hardness of the boundary layer is normally up to 1000 HV.
However, the deficiency of such layers consists in the fact that they have a large coefficient of friction and furthermore leave severe abrasion on the most commonly used opposing bodies. The cause of this deficiency is that the very hard titanium nitride needles project from the surface after the initial wear period. The local stress of the tribosystem is increased thereby until furrowing of the opposing body and, at the same time, microscopic interlockings of the titanium nitride needles and the opposing body are caused, which increase the coefficient of friction.
A further deficiency of these layers occurs during loading in an oxygenated atmosphere and preferably at higher temperatures, and expresses itself in the fact that catastrophic failures of the friction pairing can occur, especially under conditions of insufficient lubrication. The cause of this consists in that the metallic matrix between the TiN needles has a high affinity for oxygen.
In order to be able to avoid the negative effects of the TiN needles, in particular for the field of human implants, U.S. Pat. No. 5,326,362 teaches a gas nitriding process in which molecular nitrogen is diffused into the regions near the surface at a process temperature of 400xc2x0 C. to 704.4xc2x0 C. and forms a wear-resistant boundary layer by means of a solution hardening. For this purpose, the component is placed in a vacuum oven, evacuated at a pressure of 1xc2x710xe2x88x926 Torr, subsequently replenished with 1 at of nitrogen, heated to 537.7xc2x0 C., the nitrogen pressure increased to 2 at, and nitrided for several hours at 593.3xc2x0 C. After the end of the treatment, the boundary layer consists of a 0.2 xcexcm thick connecting layer of titanium nitrides, titanium carbon nitrides, titanium oxides and titanium carbooxides, and a diffusion layer that is a few xcexcm thick. The titanium nitrides located in the connective layer are significantly more finely dispersed than in laser gas alloying. Because the connective layer forms a closed layer on the surface, the loading capability of the layer in an oxygenated atmosphere and at increased temperatures is simultaneously increased thereby.
However, the deficiencies of this process consist of the fact that the coefficient of sliding friction is not sufficiently reduced and that the wear resistance at high contact pressures is not sufficient. The cause for these deficiencies results from the fact that, on the one hand, the connective layer still consists of very hard and not completely even titanium nitride needles that cause furrows in the opposing body and, on the other hand, the diffusion layer lying thereunder is too thin to be able to withstand a high local loading for a sufficiently long period of time. The latter primarily results from the fact that, in Hertzian stresses with the contact surfaces that arise in practice, the stress maximum lies under the layer. In soft base material, plastic deformations can therefore arise that cause stripping of the brittle connective layer.
The present invention provides a boundary coating construction that is biocompatible and substantially more resistant to sliding wear having a very low sliding friction coefficient for titanium and the alloys thereof and a method for producing the same.
The present invention provides a boundary coating construction that, while using the high wear resistance of titanium nitride, has a hardness depth that is at least one order of magnitude greater and that has no titanium nitride needles directly in the surface.
The present invention uses a wear-resistant, mechanically heavy duty, and lowfriction boundary layer construction for titanium or alloys thereof, consisting of a laser gas alloyed layer with deposited titanium nitride needles.
The present invention relates to a substrate of titanium and alloys thereof having a wear resistant, heavy-duty, and low-friction boundary construction, which is made of a laser gas alloyed layer having deposited titanium needles. The boundary layer comprises an outer hard amorphous carbon layer having a thickness from 200 to 400 nm, an intermediate layer having a thickness of from 5 to 50 nm and an inner laser gas alloyed layer having a thickness from 0.3 to 2.0 mm wherein the gas alloyed layer has a hardness between 600 HV0.1 and 1400 HV0.1.
It is preferred that the substrate of titanium and alloys thereof has a final roughness of less than 0.1 xcexcm before any layer deposition and that the intermediate layer has a thickness from about 5 to 20 nm.
The intermediate layer is preferably titanium.
In the solving the problem according to the present invention, an extremely thin intermediate layer is of particular significance, in particular, because of very short process times for economic reasons and because at the low thicknesses of the intermediate layer especially high adhesive strengths of the hard amorphous carbon layer can be obtained. The intermediate layer can have a thickness of 5 to 50 nm. If the final roughness of the titanium substrate before the thin intermediate layer deposition is less than 0.1 xcexcm, the intermediate layer can preferably have a thickness between 5 to 20 nm.
A preferred embodiment of the present invention is when the substrate of titanium and alloys thereof has a final roughness of less than 0.1 xcexcm before any layer deposition and the intermediate layer has a thickness from about 5 to 20 nm, thus providing a particularly good adhesion of the hard-amorphous carbon layer is achieved.
The present invention also provides a method for producing a boundary layer construction with low coefficients of friction and a very high load bearing ability. According to the invention, the method comprises:
melting a component surface to be protected in the form of tracks with a high-power laser, at an oxygen partial pressure of less than 5 ppm and in a reactive atmosphere comprising N2 and Ar, with a relative nitrogen content between 40% and 80%, thereby providing a degree of overlapping xc3x9c;
sanding the melted component to a surface roughness of at least xe2x89xa60.2 xcexcm;
cleaning the component in a high vacuum device by means of ion bombardment;
applying an intermediate layer using a laser-controlled, pulsed vacuum arc; and
applying with the laser-controlled, pulsed vacuum arc a hard amorphous carbon layer to the component.
The degree of overlapping xc3x9c is expressed in a ratio and is determined by the formula: (a-c)/a, where a is the track width, and c is the track distance. The degree of overlapping xc3x9c is 0.5xe2x89xa6xc3x9cxe2x89xa60.9. The high-power produces a laser beam having an output thickness p where p is 1xc3x97104 W/cm2xe2x89xa6p xe2x89xa62xc3x97105 W/cm2.
The invention shall be described in further detail using the following exemplary embodiment.